NMR techniques have been used for the chemical analysis of material for many years. More recently NMR techniques have been used to obtain images representing the distribution over a selected cross-sectional slice or volume of a body of a chosen quantity, e.g. the density of chosen nuclei, for example hydrogen protons, or of NMR spin relaxation time constants. Such distributions are similar to, although of different significance from, the distribution of X-ray attenuation provided by computerised tomography systems.